The manner in which central dopamine (DA)-containing neurons develop axons and establish functional connections with their target cells in the corpus striatum (CS) and frontal cortex (FCx) as well as the role of such connections in cell survival are important issues in the developmental neurobiology of these cells. This laboratory has examined these problems using an in vitro, dissociated cell-reaggregation system in which single cell suspensions prepared from specific areas of the embryonic mouse brain are allowed to reaggregate either by themselves or with cells from other specific brain regions. Only when cocultured with cells from target areas such as the CS or FCx to the DA neurons elaborate extensive axonal processes and exhibit enhanced ssurvival. Further examination of the developmental neurobiology of these DA neurons includes (1.) determining the level of specificity of the target cell-DA neuron interaction. Will DA neurons from areas of the brain which normally do not innervate target areas such as CS, i.e. DA neurons from the hypothalamus, respond to CS cells simply because they are DA neurons or alternatively do they have unique developmental cues independent of their transmitter? Specificity will also be examined from the point of view of whether medially located DA neurons primarily innervating FCx will respond to CS cells which normally receive their DA innervation from lateral DA neurons; (2.) Characterization at the ultrastructural level of the morphological connections of the DA neurons with their target cells. Antibodies to marker of neurochemically-defined target cell types e.g., cholinergic, GABAergic,, dopamine-cyclic AMP receptive phosphoprotein as well as to tyrosine hydroxylase, a marker for DA neurons, allows us to examine the development of synapses between DA cells and neurochemically-defined target cells; (3.) Study of the functional organization of the DA neurons in relation to target cells by assessment of pharmacological responses; (4.) Isolation and characterization of macromolecule(s) present upon target cells which are responsible for the increase in DA-cell survival and axonal proliferation. Cell fusion techniques will be used to obtain homogeneous cell hybrids derived from target cells of known neurochemical type (e.g., cholinergic, GABAergic, glial). These cells will then be exploited for the characterization of such factors utilizing immunological techniques. The DA neuronal systems are important sites of action for psychopharmacologic agents and these experiments should yield further information on their normal development and its modification by drugs as a function of the cellular and chemical environment in which such development occurs.